This invention relates to a process for the oxidation of ketones to esters. The process involves contacting the ketone with hydrogen peroxide and a catalyst which comprises a tin substituted molecular sieve at oxidation conditions to form the corresponding ester.
Esters and lactones (cyclic esters) have various uses in and of themselves and also can be intermediates in the synthesis of antibiotics, steroids, phermones, fragrances and monomers. In 1899 Adolph Baeyer and Victor Villiger first reported the oxidation of menthone and tetrahydrocarvone to the corresponding lactones. The reaction was carried out using monopersulfuric acid, which was the most powerful oxidant known at that time. There has been considerable interest in the Baeyer-Villiger reaction in academia and in industry with numerous papers being published. See, e.g., G. Strukul, Angew. Chem. Int. Ed., 37,11-98 (1998).
The reaction is usually carried out with organic per-acids. When the oxidant is hydrogen peroxide, there are reports of using transition metal catalysts for the Baeyer-Villiger reaction. For example, Jacobson et al., J.Chem. Soc. Chem. Comun., 888, (1978) and in Inorg. Chem., 17, 3055 (1978), have disclosed the use of molybdenum(VI) peroxo-complexes as catalysts in combination with 98% hydrogen peroxide as the oxidant. W. A. Herrmann, et al., in J. Mol. Catal., 94, 213 (1994) have disclosed that the di-peroxo complex of methyl trioxorhenium is also active for the Baeyer-Villiger reaction. In his paper, Strukul also reports on the use of platinum complexes to carry out the oxidation of ketones in conjunction with 35% hydrogen peroxide. Finally, A. Bhaumik et al., in Catal. Lett., 40, 47 (1996) discloses the use of titanium silicalite (TS-1) as a catalyst for the oxidation of ketones in conjunction with hydrogen peroxide. However, the use of TS-1 gave selectivities to the ester of below 50% with hydroxycarboxylic acids being the major by-products.
In contrast to the work disclosed above, applicants have developed a process for converting ketones to esters or lactones, which uses as the catalyst a tin substituted molecular sieve in conjunction with hydrogen peroxide. The catalyst has an empirical formula on an anhydrous basis of:
(MwSnxTiySi1xe2x88x92xxe2x88x92yxe2x88x92zGez)O2
where M represents a metal having a +3 valence such as Al or B and xe2x80x9cwxe2x80x9d is the mole fraction of M and varies from 0 to about 2x. The value of xe2x80x9cxxe2x80x9d can be from about 0.001 to about 0.1 while xe2x80x9cyxe2x80x9d and xe2x80x9czxe2x80x9d have, respectively, values of 0 to about 0.1 and 0 to about 0.08. The catalysts of this invention have been found to have higher conversions and virtually exclusive selectivity to the lactones.
An object of the present invention is the conversion of ketones and especially cyclic ketones to esters and especially lactones. Accordingly, one embodiment of the invention is a process for the oxidation of a ketone to an ester comprising contacting a ketone with hydrogen peroxide and a catalyst at oxidation conditions to provide the corresponding ester, the catalyst comprising a molecular sieve having an empirical formula on a calcined and anhydrous basis of:
(MwSnxTiySi1xe2x88x92xxe2x88x92yxe2x88x92zGez)O2
where M is a metal having a +3 valence, xe2x80x9cwxe2x80x9d is the mole fraction of M and varies from 0 to about 2x, xe2x80x9cxxe2x80x9d is the mole fraction of tin and varies from about 0.001 to about 0.1, xe2x80x9cyxe2x80x9d is the mole fraction of titanium and varies from zero to about 0.1 and xe2x80x9czxe2x80x9d is the mole fraction of germanium and varies from zero to less than about 0.08 and characterized in that the composition has the characteristic x-ray diffraction pattern of zeolite beta, and when xe2x80x9cwxe2x80x9d, xe2x80x9cyxe2x80x9d and xe2x80x9czxe2x80x9d are all zero, then the molecular sieve is amorphous with short range order or has the characteristic x-ray diffraction pattern of zeolite beta.
This and other objects and embodiments of the invention will become more apparent after a detailed description of the invention.
As stated, the present application deals with a process (known as the Baeyer-Villiger reaction) in which ketones are converted to esters. It is preferred to convert cyclic ketones to cyclic esters which are generally called lactones. One essential part of this process is a catalyst which comprises a tin containing molecular sieve having the characteristic x-ray diffraction pattern of zeolite beta and an empirical formula on a calcined and anhydrous basis of:
(MwSnxTiySi1xe2x88x92xxe2x88x92yxe2x88x92zGez)O2
xe2x80x9cxxe2x80x9d is the mole fraction of tin and varies from about 0.001 to about 0.1, xe2x80x9cyxe2x80x9d is the mole fraction of titanium and varies from zero to about 0.1 and xe2x80x9czxe2x80x9d is the mole fraction of germanium and varies from zero to less than about 0.08. However, when xe2x80x9cwxe2x80x9d, xe2x80x9cyxe2x80x9d and xe2x80x9czxe2x80x9d are all zero, then the molecular sieve is either amorphous with short range order or has the zeolite beta structure. The M metals which can be used include but are not limited to aluminum, boron, gallium, and iron; and xe2x80x9cwxe2x80x9dis the mole fraction of M and varies from 0 to about 2x. These molecular sieves have a microporous three dimensional framework structure of at least SiO2 and SnO2 tetrahedral units, and a crystallographically regular pore system.
These molecular sieves are prepared using a hydrothermal crystallization process in which a reaction mixture is prepared by combining reactive sources of tin, silicon, an organic templating agent, optionally germanium, optionally titanium, optionally a M metal, a fluoride or hydroxide source, optionally hydrogen peroxide and water. The sources of silicon include but are not limited to colloidal silica, amorphous silica, fumed silica, silica gel and tetraalkylorthosilicate. Sources of tin include but are not limited to tin halides, tin alkoxides, tin oxide, metallic tin, alkaline and alkaline earth stannates and alkyl tin compounds. A preferred source is tin tetrachloride. Examples of tin alkoxides include tin butoxide, tin ethoxide and tin propoxide. The organic templating agents include, without limitation, tetraalkylammonium ions such as tetraethylammonium ions, aza-polycyclic compounds such as 1,4-diazabicyclo 2,2,2, octane; dialkyldibenzylammonium ions such as dimethyldibenzyl ammonium ion and bis-piperidinium ions such as 4,4xe2x80x2 trimethylene bis (N-benzyl N-methyl piperidinium) ion. These ions may be added as the hydroxide or halide compounds. Germanium sources include germanium halides, germanium alkoxides and germanium oxides. Titanium sources include titanium alkoxides and titanium halides. Preferred titanium alkoxides are titanium tetraethoxide, titanium isopropoxide and titanium tetrabutoxide. When M is aluminum, the sources of aluminum include but are not limited to aluminum oxides, such as pesudo-boehmite, aluminum alkoxides such as aluminum isopropoxide, sodium aluminate and aluminum trichloride, with pseudo-boehmite and aluminum alkoxides being preferred. Sources of boron, gallium and iron include oxides, hydroxides, alkoxides, nitrates, sulfates, halides, carboxylates and mixtures thereof. Representative compounds include without limitation boron alkoxides, gallium alkoxides, iron (II) acetate, etc.
The reaction mixture will also contain either a fluoride source such as hydrofluoric acid or ammonium fluoride or a hydroxide source such as sodium hydroxide or potassium hydroxide. The hydroxide source may also be added by using the hydroxide compound of the templating agent. Water is also added to the mixture and optionally hydrogen peroxide.
Generally, the hydrothermal process used to prepare the tin containing molecular sieves involves forming a reaction mixture, using the sources stated above, which is expressed by the formula:
SiO2:kM2O3:aR2O:bSnO2:cGeO2:dTiO2:eF:fH2O2:gH2O
where xe2x80x9ckxe2x80x9d has a value from zero to about 0.1, xe2x80x9caxe2x80x9d has a value from about 0.06 to about 0.5, xe2x80x9cbxe2x80x9d has a value from about 0.001 to about 0.1, xe2x80x9ccxe2x80x9d has a value from zero to about 0.08, xe2x80x9cdxe2x80x9d has a value from 0 to about 0.1, xe2x80x9cexe2x80x9d has a value from about 0.1 to about 2, xe2x80x9cfxe2x80x9d has a value from zero to about 0.5 and xe2x80x9cgxe2x80x9d has a value from about 4 to about 50. The reaction mixture is prepared by mixing the desired sources of tin, silicon, optionally titanium, optionally germanium, optionally a M metal, an organic templating agent, water, optionally hydrogen peroxide and a fluoride or hydroxide source in any order to give the desired mixture. It is also necessary that the pH of the mixture be in the range of about 6 to about 12 and preferably in the range of about 7.5 to about 9.5. If necessary the pH of the mixture can be adjusted by adding HF, NH4F, NaOH, KOH, etc. Hydrogen peroxide may be added in order to form a complex with titanium and maintain it in solution.
Having formed the reaction mixture, it is next reacted at a temperature of about 90xc2x0 C. to about 200xc2x0 C. and preferably 120xc2x0 C. to about 180xc2x0 C. for a time of about 2 days to about 50 days and preferably from about 10 days to about 25 days in a sealed reaction vessel under autogenous pressure. After the allotted time, the mixture is filtered to isolate the solid product, which is washed with deionized water and dried in air.
In order to promote crystallization of the zeolite beta phase, it is preferred to add zeolite beta crystals as seeds to the reaction mixture. These crystals can be added as a dry solid, a suspension in an appropriate liquid, e.g., water, alcohol or a preorganized gel, i.e., a gel which contains nuclei. A preferred zeolite beta seed is one prepared according to the teachings of Spanish Patent Application No. P9501552.
The isolated molecular sieve is characterized in that it has the x-ray diffraction pattern characteristic of zeolite beta which includes at least the peaks and intensities presented in Table A. The intensity presented in Table A is a relative intensity which is obtained by relating the intensity of each peak (I) to the strongest line (Io). The intensity is given by the equation 100xc3x97I/Io and are represented by vs, s, m and w, where these are defined as: vs=80-100; s=60-80; m=15-60 and w=0-15.
When the only elements which are present in the framework are Sn and Si, the molecular sieve can either have the zeolite beta structure or be amorphous with short range order. The amorphous composition has the characteristics of the material described in U.S. Pat. No. 3,556,725.
As synthesized, the molecular sieves of this invention will contain some of the organic templating agent and fluoride ions in the pores of the sieve. In order to activate the zeolite, i.e., active for adsorption or catalytic reactions, it is necessary to remove the organic template and fluoride. This is generally accomplished by calcining the molecular sieve at a temperature of about 300xc2x0 C. to about 1000xc2x0 C. for a time sufficient to remove substantially all the organic template and fluoride which usually is about 1 to about 10 hrs.
As stated, the molecular sieves described above have very good activity as catalysts for the oxidation of ketones to esters. Examples of ketones which can be used in the instant process include, without limitation, alkyl ketones, cyclic ketones, alkyl substituted cyclic ketones, aryl ketones and alkyl aryl ketones. Specific examples include cyclopentanone, cyclohexanone, methyl cyclopentanone, methyl cyclohexanone, n-pentylcyclopentanone, t-butyl cyclohexanone and adamantanone.
The process involves contacting the ketone with a catalyst (as described above) and hydrogen peroxide at oxidation conditions. Oxidation conditions for the instant process include a temperature of about 20xc2x0 C. to about 120xc2x0 C., and preferably about 40xc2x0 C. to about 90xc2x0 C., a pressure of about atmospheric to about 400 kPa, and a contact time of about 10 min. to about 24 hours and preferably a time of about 1 hour to about 12 hours. As stated, it is also required to use hydrogen peroxide. Hydrogen peroxide can be obtained as a solution having a concentration of about 3% to about 70% of H2O2 by weight of the solution. Any of these commercially available solutions can be used in the instant process with a 35% solution being preferred. The ketone may be present neat or it can be mixed with a solvent, with the use of a solvent being preferred. Examples of solvents which can be used include but are not limited to alcohols, ethers, acetals and acetonitrile.
The process can be carried out in either a batch mode or a continuous mode. In a batch mode, the catalyst, ketone and H2O2 are mixed in a suitable reactor preferably with stirring at the desired temperature for a time of about 10 min. to about 24 hours and preferably a time of about 1 hour to about 12 hours. Whether a batch or continuous mode is used, the molar ratio of H2O2 to ketone can vary from about 2:1 to about 0.1:1 and preferably from about 1:1 to about 0.3:1. In a continuous mode, the catalyst can be used as a fixed bed, fluidized bed, moving bed, or any other configuration known to one of ordinary skill in the art. When a fixed bed is used, the ketone and hydrogen peroxide can be flowed in either an upflow or downflow direction. The H2O2 and ketone can be injected separately or can be premixed and then injected into the reactor. Regardless of how the reactants are introduced and the type of bed being used, the reactants are flowed through the reactor at a liquid hourly space velocity (LHSV) of about 0.05 to about 10 hrxe2x88x921 in order to insure adequate contact time between the reactants and the catalyst. Finally, regardless of whether a batch or continuous process is used, the products, reactants, and any formed byproducts are separated by means well known in the art.